The primary goal of this Pathway to Independence proposal is to gain a full understanding of the roles of IL-12 and IL-23 signaling in human mycobacterial immunity. Mendelian susceptibility to mycobacterial disease (MSMD) is a genetic condition predisposing children to severe disease upon infection by weakly virulent mycobacteria, including the BCG vaccine. Although MSMD is rare, the study of this disease has the power to elucidate the basic immunological mechanisms of immunity to the more virulent Mycobacterium tuberculosis. In the proposed research program, I will use knowledge gleaned from ?natural experiments?, that is, inborn errors of mycobacterial immunity, to further our general understanding of mycobacterial pathogenesis. In particular, I will functionally characterize novel bi-allelic mutations I have recently discovered in IL12RB2 and IL23R, genes encoding the receptor chains that may pair with IL-12R?1 and are required for cellular responses to IL-12 and IL-23, respectively (Aim 1). These mutations are present in 2 kindreds with multiple MSMD-affected children. The receptors for IL-12 and IL-23 are considered components of either the canonical Th1 or Th17 T cell pathways, and are known to play critical roles in IFN-? production, IL-17 production, or both. Therefore, I will carry out detailed mechanistic studies to understand the differential impacts of these novel monogenic lesions on the development and function of Th1 and Th17 cells (Aim 2). Finally, I will test the hypothesis that defects in IL-12 or IL-23 signaling cause MSMD in these patients, and seek to identify additional monogenic causes of MSMD in immunologically related genes in amongst >500 other MSMD patients (Aim 3). In the mentored K99 phase of the award, I will benefit from the unique learning environment and clinical resources of Dr. Jean-Laurent Casanova?s laboratory at the Rockefeller University (RU). Dr. Casanova?s lab is the world leader in monogenic dissection of infectious diseases, and interrogating susceptibility to mycobacteria is the lab?s longest-running project. While working in Dr. Casanova?s lab, I will enjoy unparalleled access to physicians and patients around the world, and the close counsel of many experienced colleagues. I will also gain experience with cutting-edge techniques including whole exome and whole genome sequencing, RNAseq, imaging flow cytometry, and multiplex cytokine arrays. A critical part of my career development plan is the scientific collaboration and career development advice offered by my Advisory Committee, consisting of Dr. Casanova, Dr. Luigi Notarangelo (NIAID/NIH), and Dr. Stuart Tangye (Garvan Institute, Australia), all of whom have pledged to support me in my proposed research and in the development of my independent career. In all, the proposed plan will fulfill my short-term scientific goal of elaborating the single-gene lesions responsible for mycobacterial susceptibility in children, and my long-term goal of attaining an independent role investigating the host genetic determinants of immunity to pathogens.